It has already been proposed heretofore to provide, in association with extruder means for producing a relatively long flat (plate-shaped) synthetic-resin strand, a roller assembly comprising a plurality of pressing rollers in a roller stand through which the strand passes. The roller stand is generally stationary and the pressing or calibrating rollers are neither heated or cooled. As a result, the synthetic-resin strand undergoes warping, bulging and like deformation upon cooling or subsequent to cooling, this being especially the case when relatively thick and broad plates are to be produced, i.e. when the strand is relatively thick and wide.